In recent years, as the use of the Internet increases, electronic apparatuses that use CPUs running at high clock frequencies in the sub-microwave band (0.3 to 10 GHz), electronic apparatuses that use high frequency bus, and telecommunication apparatuses that utilize radio waves have been increasing, such as personal computers, home appliances having information processing functions, wireless LAN, bluetooth-equipped apparatuses, optical module, mobile telephones, mobile information terminals and intelligent road traffic information system. This trend leads to a society of ubiquitous computing that requires devices of higher performance with high-speed digital information processing function and low-voltage driving. However, as such apparatuses become popular, concerns have been increasing on the problems related to the electromagnetic interference such as malfunctions of the apparatus that emit electromagnetic radiation or other apparatuses and health threats to the human body. For this reason, such an apparatus is required to minimize the emission of unnecessary electromagnetic radiation so as not to affect its own operation and that of other apparatuses and not to cause adverse effect on the human body, and to operate without malfunction when subjected to electromagnetic radiation emitted by other apparatuses. Measures to prevent such electromagnetic interference include the use of an electromagnetic radiation shielding material that reflects electromagnetic radiation and the use of an electromagnetic absorbing material.
As the means for preventing electromagnetic interference between electronic apparatuses, electromagnetic radiation shielding material is provided on the surface of the housing of the electronic apparatus or between electronic apparatuses so as to block electromagnetic radiation (inter-system EMC). As a means for preventing electromagnetic interference within an electronic apparatus, electronic components and circuits are covered with electromagnetic radiation shielding material so as to prevent the electronic components and circuits from interfering with each other and resulting in malfunction, and suppressing the processing speed from decreasing and signal waveform from being distorted (intra-system EMC).
It has also been proposed to suppress the generation of electromagnetic noise by providing electromagnetic noise suppressing measures to electronic components that are the sources of the electromagnetic noise or to suppress the interference between signals thereby to improve the transmission characteristic in near-field environments such as within an electronic apparatus (micro EMC).
Electronic apparatuses and electronic components are recently required to have higher performance and be smaller and lighter in weight, and the electromagnetic noise suppressor used in these apparatuses or components is also required to have high electromagnetic noise suppressing effects in a high-frequency band such as sub-microwave band, become smaller and lighter in weight, and be easy to carry out by the work which takes measures with electromagnetic noise suppressing measures.
Conventional electrically conductive shield strengthens electromagnetic coupling due to the reflection from unnecessary radiation source. Therefore, it is said to be effective to suppress the unnecessary radiation source by making use of magnetic loss of magnetic material, namely the imaginary part of complex permeability μ″. Japanese Patent Application, First Publication No. Hei 9-93034 discloses an electromagnetic radiation absorbing material made by adding about 95% by weight of such a magnetic powder to an organic binding agent as thickness of flaked powder of soft magnetic material is smaller than the skin depth, having sufficiently high aspect ratio with the magnetic material turned to be non-conductive. The electromagnetic radiation absorbing material is said to have high electromagnetic radiation absorbing property and flexibility. In this example, an electromagnetic radiation absorbing material provided with a backing made of a copper plate is used for evaluation, and thickness of the electromagnetic radiation absorbing material and the copper plate used for measurement combined is 2 mm.
However, thickness of the electromagnetic radiation absorbing material and the copper plate combined is 2 mm and sheet thickness of the electromagnetic radiation absorbing material excluding the copper plate is as thick as 1 mm or more, and is heavy because 95% by weight of the electromagnetic radiation absorbing material consists of ferromagnetic material such as iron. Therefore, it cannot be said that weight reduction has been achieved. It is also not sufficiently robust and flexible since its content of organic binding agent is small. Moreover, the flaked powder of soft magnetic material is expensive because it requires tedious processes to form the soft magnetic material in flakes and making the surface not electrically conductive. The electromagnetic radiation absorbing material that uses the flaked powder of soft magnetic material in a large amounts also is expensive and cannot satisfy the needs of industry.
Japanese Patent Application, First Publication No. Hei 9-181476 discloses an electromagnetic radiation absorbing material made by forming a layer of a ferromagnetic element and ceramic element by magnetron sputtering on a substrate, and annealing at a low temperature so as that ultra-fine crystal of ferromagnetic material precipitates in the ceramic phase of high resistivity thereby achieving isolation. It is claimed that the electromagnetic radiation absorbing material has high electrical resistance in a high-frequency band from 100 MHz to 10 GHz, capability to suppress the reflection of electromagnetic radiation due to eddy current and a large value of imaginary part of complex permeability μ″, thus resulting in high electromagnetic radiation absorbing property.
It is said that the electromagnetic radiation absorbing material requires heat treatment at a high temperature in order to form the ultra-fine crystal of ferromagnetic material in the ceramic phase. In this example, a film is formed from ceramic and ferromagnetic elements on a glass slide by a RF magnetron sputtering method, and heat treatment is applied at a temperature from 200 to 350° C., thereby forming the ultra-fine crystal of ferromagnetic material. It cannot be avoided to use an organic film that has high heat resistance as the organic film of this electromagnetic radiation absorbing material, although the ceramic phase and ultra-fine crystal of ferromagnetic material phase are formed on the organic film. Since the organic film which has high heat resistance is expensive, the electromagnetic radiation absorbing material that uses it is also expensive. Moreover, even when the ultra-fine crystal of ferromagnetic material is formed on the organic film having high heat resistance, there is a significant difference in the thermal expansion coefficient between the organic film and the ceramic phase that causes cracks, resulting in a material far from being flexible or tough.
As for the electromagnetic noise suppressor, it has been proposed to make a thin electromagnetic noise suppressor that has electromagnetic noise suppressing effect in the sub-microwave band by ferrite plating technology (Masaki ABE et al., Proceedings of the 131st Conference, pp 25-31, Jul. 4, 2003; The Magnetics Society of Japan).
The electromagnetic noise suppressor based on the ferrite plating technology is made by applying a reaction solution of chlorides of iron, nickel and zinc and an oxidation liquid comprising sodium nitrate and ammonia acetate on a polyimide sheet placed on a rotary substrate, so as to form a ferrite compound film having a thickness of 3 μm by plating by a spin spray process. This electromagnetic noise suppressor has, despite smaller thickness, electromagnetic noise suppressing effect similar to that of the conventional electromagnetic noise suppressor made in a sheet 50 μm thick by dispersing the fine particles of flake-shaped metal in the organic binding agent, and is said to be advantageously applied to small electronic apparatuses.
However, thickness of the ferromagnetic layer of this electromagnetic noise suppressor is from 3 to 11 μm and power loss of the electromagnetic noise suppressor at 1 GHz is about 0.2 even when formed with a large thickness, resulting in insufficient electromagnetic noise suppressing effect in low frequency portion of the sub-microwave band. When the thickness of the ferromagnetic material increases, sufficient robustness and flexibility cannot be achieved since the ferrite layer formed on the polyimide is hard and does not contain organic binding agent. Moreover, since it is made in a wet process, it requires tedious processes such as removal of impurities and drying, and does not satisfy the needs of industry.
It has also been proposed to make an electromagnetic noise suppressor comprising a ultra-fine crystal of a ferromagnetic material film containing alumina ceramics phase and ultra-fine crystal phase of ferromagnetic material of iron or cobalt (Shigehiro OHNUMA et al., Proceedings of the 131st Conference, pp 17-24, Jul. 4, 2003; The Magnetics Society of Japan). This proposal relates to an electromagnetic noise suppressor having a thickness of 1 μm made by forming layer of ferromagnetic element and ceramic element by high-frequency magnetron sputtering on a substrate, annealing at a low temperature so that ultra-fine crystals of ferromagnetic material precipitate in the ceramic phase of high resistivity and dividing by forming a slit so as to increase resistivity of the film further, which is claimed to have high noise suppressing effect.
However, this electromagnetic noise suppressor has electromagnetic radiation absorbing property similar to that of the conventional electromagnetic noise suppressor made in a sheet 50 μm thick by dispersing the fine particles of flake-shaped metal in the organic binding agent and power loss of about 0.2 at 1 GHz, resulting in insufficient electromagnetic noise suppressing effect in the effective frequency band. Also, it is necessary to apply heat treatment in order to form the ultra-fine crystals of ferromagnetic material in the ceramic phase and a micro slit must be formed by photolithography or by means of a dicing saw in order to increase the resistivity of the thin magnetic film, making the process tedious. Also because it is a thin ceramic film, it is prone to cracks, resulting in a material which is far from being flexible or robust.
Electrical and electronic apparatuses are required to have flame retarding property (in accordance with UL94 V-0, V-1 or VTM-0, VTM-1) to ensure safety, and the electromagnetic noise suppressor used in such an apparatus is also required to have flame retarding property (in accordance with UL94 V-0, V-1 or VTM-0, VTM-1). The UL refers to standards specified by Underwriters Laboratory Inc. of the United States for the safety of electrical apparatuses, and UL94 is a standard relating to flame retarding property. Flame retarding property specified in UL94 V-0, V-1 or VTM-0, VTM-1 will hereinafter be referred to simply as flame retarding property.
Japanese Patent Application, First Publication No. 2000-196281 discloses an electromagnetic radiation absorbing material that has flame retarding property made by applying a coating material containing a high-polymer binding agent, a soft magnetic material powder and a phosphorus-based flame retarding agent on a supporting body thereby forming an electromagnetic radiation absorbing layer.
However, in the case in which the soft magnetic material powder is used as the electromagnetic radiation absorbing material, a large quantity of the soft magnetic material powder must be used in order to achieve sufficient effect of absorbing electromagnetic radiation, the amount being 100 parts by weight for 5 to 12 parts by weight of the high-polymer binding agent. Also, in the case in which the soft magnetic material powder is used as the electromagnetic radiation absorbing material, the electromagnetic radiation absorbing layer must be made thick in order to achieve sufficient effect of absorbing electromagnetic radiation. As a result, the electromagnetic radiation absorbing material becomes heavy because the electromagnetic radiation absorbing layer has a high specific gravity and is thick. Also there has been a problem in that it is difficult to reduce the space requirement because the electromagnetic radiation absorbing layer is provided on the support material and is thick. Moreover, since the soft magnetic material powder is a metal powder, it can easily generate heat and ignite. Therefore, a large amount of the flame retarding agent must be added in order for the electromagnetic radiation absorbing material to exhibit sufficient flame retarding property. Also, because the electromagnetic radiation absorbing material consists mostly of soft magnetic material powder with a small proportion occupied by the polymer binding agent, it is less flexible and is brittle.
Japanese Patent Application, First Publication No. 2002-84091 proposes an electromagnetic radiation absorbing sheet formed by stacking an electromagnetic radiation absorbing material consisting of ferrite powder or soft magnetic material powder mixed in a resin as an electromagnetic radiation absorbing material that has flame retarding property and a flame retarding material.
However, in the case of a ferrite powder or the soft magnetic material powder is used as the electromagnetic radiation absorbing material, a large amount of the material must be used in order to achieve sufficient effect of absorbing electromagnetic radiation, the amount being about 90% by weight of the electromagnetic radiation absorbing material. Also, when ferrite powder or the soft magnetic material powder is used as the electromagnetic radiation absorbing material, the electromagnetic radiation absorbing layer must be made thick in order to achieve sufficient effect of absorbing electromagnetic radiation. As a result, the electromagnetic radiation absorbing material is heavy because the electromagnetic radiation absorbing layer has a high specific gravity and is thick. Also, there has been a problem in that it is difficult to reduce the space requirement because the electromagnetic radiation absorbing material has a large thickness. Moreover, since ferrite powder and soft magnetic material powder can easily generate heat and ignite, it may be difficult to prevent it from burning simply by laminating a flame retarding material. Also, because the electromagnetic radiation absorbing material consists mostly of ferrite powder and soft magnetic material powder with a small proportion occupied by a resin, it is less flexible and is brittle.
Japanese Patent Application, First Publication No. Hei 7-212079 discloses an electromagnetic interference suppressor that comprises an electrically conductive support member and an insulating soft magnetic material layer provided on at least one surface of the electrically conductive support member, where the insulating soft magnetic material layer contains soft magnetic material powder and an organic binding agent.
This electromagnetic interference suppressor has a large thickness and contains much magnetic material in the entire region of the insulating soft magnetic material layer, and is therefore heavy. It is also not sufficiently tough and flexible, and does not meet the requirements for high-density packaging.
With the background described above, an object of the present invention is to provide an electromagnetic noise suppressor that has high electromagnetic noise suppressing effect in the sub-microwave band, requires small installation space and is light in weight, articles such as electronic components and printed wiring board that are provided with electromagnetic noise suppressing means and a manufacture method that these can be manufactured easily.
Another object of the present invention is to provide an electromagnetic noise suppressor that is flexible and has high strength.
Still another object of the present invention is to provide an electromagnetic noise suppressor that has a sufficient flame retarding property.
A further object of the present invention is to provide an electromagnetic noise suppressor that also has an electromagnetic radiation shielding property.